The invention relates to a device for the simultaneous monitoring, sealing and electrical insulation of an electrode and to a method for supplying current to a reactor.
In particular, the invention relates to the current supply of chemical reactors which comprise a reactor chamber in which certain reaction gases are heated to a temperature by means of heating elements, the heating elements being heated by direct current flow. To this end, the heating elements are made of an electrically conductive material and are connected to a current supply. The invention likewise relates to the current supply of so-called Siemens reactors.
For the deposition of polysilicon according to the Siemens process, highly pure elemental silicon is deposited from the gas phase on the surface of silicon rods. In this case, elemental silicon is deposited from the gas phase in a deposition reactor on the surface of a thin silicon rod heated to from 900 to 1200° C. from a mixture of hydrogen and halosilanes (for example trichlorosilane) or a silicon compound containing hydrogen.
The silicon rods are held in the reactor by special electrodes, which generally consist of highly pure electrographite. In each case, two thin rods with different voltage poling on the electrode holders are connected at the other thin rod end by a bridge to form a closed circuit. Electrical energy for heating the thin rods is supplied via the electrodes and their electrode holders. A mixture of hydrogen and halosilanes is supplied through inlet nozzles on the bottom plate of the deposition reactor. The halosilanes decompose on the surface of the thin rods. The diameter of the thin rods thereby grows. After reaching a desired setpoint diameter of the silicon rods, the deposition process is terminated and the hot silicon rods are cooled and extracted.
In this case, particular importance is attached to the protection of the electrode and of the seal surrounding the electrode holder. Since there is a trend towards ever longer and heavier rods in shorter deposition cycles, the arrangement and shape of the electrode seal protective bodies and the material of the seal to be protected are specifically of importance. This is because the possible perturbations influencing yield and/or quality in the polysilicon deposition process can be avoided by an optimized arrangement. These possible perturbations include electrical failures, for example due to ground faults, during the deposition and a sealing defect of the reactor due to the feedthrough of the current-carrying electrodes in the reactor bottom of the CVD reactor.
Furthermore, very different requirements are placed on the silicon rods and the deposition process, and therefore on the electrodes and their protection, depending on the subsequent use of the silicon rods produced in this way. If the polycrystalline silicon is subsequently used in silicon chunks for solar and electronic applications, for example, the silicon rods must not collapse during or after the deposition process in the deposition reactor or be contaminated by emerging extraneous substances, for example from seal materials touching the product.
WO Patent 2010/083899 A1 discloses an electrode protection device according to the prior art. In this case, thin rods are described in a graphite adapter, which engages in a graphite clamping ring which itself interacts via a quartz ring with the bottom plate of the CVD reactor for producing polycrystalline silicon according to the monosilane process.
The perturbations influencing the possible yield and/or quality in the polysilicon deposition process include electrical failures due to ground faults during the deposition. This perturbation gives rise to a difference between the real output and the maximum possible output.
In the prior art, attempts have been made to resolve this problem by sealing and insulating the electrode holders.
It is known from WO 2010/083899 A1 to shield the seals of the electrode holders against thermal loading by means of protective rings made of quartz.
DE 23 28 303 A1 describes a device for producing rods and tubes of silicon by depositing the relevant semiconductor material from the gas phase on the lateral surface of a heated elongate support, in particular made of silicon or graphite, consisting of a reaction vessel which comprises a metal base plate and is provided with at least one electrode which is used for holding one end of the elongate support and for heating the support and is electrically insulated and fed in a leaktight manner through the base plate, characterized in that a first electrode part consisting of metal is fastened in the base plate with the interposition of a sealing layer of inert insulating material, in particular tetrafluoropolyethylene, and comprises a projection extending into the reaction space, on which is replaceably seated a further electrode part consisting of metal or carbon, which part comprises the fitting surface intended for receiving and holding the support on its free surface.
A first part of the electrode holder, consisting of metal, is thus fastened in the bottom plate with the insertion of a sealing layer made of inert insulating material.
JP 2009-221058 A2 discloses a seal and insulation making use of a special zirconium ceramic, of flexible graphite, and coated O-rings as a seal. Such materials have refractory stability and make it possible to seal the gap between the electrodes and the base plate.
WO 2010/068849 A1 describes improved thermal insulation in the region of the feedthrough of the electrode holder through the bottom plate using a metal body, which is provided with an insulating surface coating.
However, the devices known to date do not present sufficient protection of the seal of the electrode holder. This has the effect that the probability of failure due to corrosive effects and ground faults is increased. Furthermore, sufficient protection of the seal against corrosion and therefore discharge of substances which influence the product quality (in particular dopants) has not yet been found.
DE 3024320 A1 discloses a device for the high-temperature treatment of gases, consisting of a thermally insulated housing having gas inlet and gas outlet openings as well as inert resistance heaters, which are arranged between these openings and are heated by direct current flow. The heating of the electrically conductive resistor bodies is preferably carried out using a star circuit in a symmetrical polyphase AC system. Individual heater groups can in this case be regulated differently to one another, i.e. heated differently by electrical current flow.
An example of such a device is a reactor for converting silicon tetrachloride into trichlorosilane.
Trichlorosilane is used in the Siemens process for producing polycrystalline silicon. In this case, silicon is deposited on heated thin rods in a reactor. Trichlorosilane in the presence of hydrogen is used as process gas. During the conversion of trichlorosilane (disproportionation) into deposited silicon, large amounts of silicon tetrachloride are formed.
By reaction with hydrogen and oxygen at elevated temperatures in combustion chambers, for example, highly disperse silica can be produced from silicon tetrachloride.
The economically most advantageous use of silicon tetrachloride is, however, conversion into trichlorosilane. This is carried out by reacting silicon tetrachloride with hydrogen to form trichlorosilane and hydrogen chloride. It is thereby possible to produce trichlorosilane from the silicon tetrachloride byproduct formed during the deposition, and to feed this trichlorosilane back into the deposition process in order to produce elemental silicon.
The conversion of silicon tetrachloride with hydrogen into trichlorosilane is typically carried out in a reactor at high temperatures, at least 600° C., ideally at least 850° C. and at a pressure of 0-30 bar.
To this end, electrical current is fed directly through the conductive heating elements and the electrical energy is converted into heat in the heating element by the electrical resistance.
The heating elements usually consist of carbon-containing materials, for example graphite, CFC, silicon carbide or similar materials.
It is known that the carbon-containing installed reactor components are exposed to chemical attack during the conversion of silicon tetrachloride in the presence of hydrogen.
The chemical attack of these carbon-containing components gives rise to carbon deposits, which are electrically conductive and can therefore lead to ground faults of the electrical energy network. Furthermore, chemical attack can also lead to component failures of the internal components, which then entails flaking or splitting of small parts which can in turn lead to ground faults. The problem with these ground faults is that they cannot be distinguished from damage to the electrode seals. If there is damage to the electrode seal, the current supply and the reactor have to be taken out of operation since continued operation can lead to a sealing defect and a reaction gas leakage, which needs to be prevented.
Conventionally, nonmetallic or electrically nonconductive seals are used for electrodes, since these fulfill a twofold function, namely electrical insulation of the electrode from the reactor wall and a sealing function. However, the temperatures in these reactors are so high that there are scarcely any electrically insulating as well as chemically stable materials which fulfill the function of electrical insulation and a pressure-tight seal.
Furthermore, attempts may be made to protect the seals from excessively high temperatures by additional installed components in the reactor.
In spite of this, seal damage triggered by unintended electrical currents from one electrode through the reactor wall to another electrode cannot be ruled out by such measures.
The object of the present invention was based on this problem.